As one ionization process in mass spectrometry capable of speedily and accurately diagnosing pathogenic germs and bacteria, matrix assisted laser desorption/ionization (MALDI) process is known.
The MALDI process is a process of ionizing a sample by mixing a sample in advance in a material (hereinafter called “matrix”) that is likely to absorb laser light and to be ionized and irradiating a resultant mixture with laser light, in order to analyze an analyte that is less likely to absorb laser light or is susceptible to damage by laser light.
In a mass spectrometer utilizing the MALDI process, generally, a plate made of metal (hereinafter, called a “sample mounting plate”) called a target plate on which a matter obtained by mixing an analyte and a matrix in advance and liquefying the mixture by a solvent (hereinafter, the matter is called a “sample”, and a matter that is liquid at the time of dripping and is dried to be crystallized is also called a “sample”) is mounted is placed in the spectrometer, and the sample mounted on the sample mounting plate is irradiated with laser light for a predetermined time to desorb and ionize the analyte. In this event, voltage is applied to the sample mounting plate made of metal to place the desorbed/ionized analyte in an electric field, thereby making the desorbed/ionized analyte easily fly toward an electrode for acceleration.
The sample mounting plate has a plurality of sample mounting regions (hereinafter, called “sample mounting spots”) for mounting the sample thereon, and the sample mounting plate is placed in the mass spectrometer after a plurality of samples to be measured are respectively dripped to predetermined sample mounting spots and dried (crystallized), and the plurality of samples are irradiated with laser by moving the sample mounting plate.
In the MALDI process, it is important that the crystals deposit as uniformly as possible in the sample mounting spots, the analytes are appropriately desorbed/ionized, and appropriately ionized and accelerated without charge-up of the sample, and many suggestions regarding these analysis techniques have been made.
Regarding improvement of crystallization of a sample at a sample mounting spot and ionization of an analyte, for example, suggestion disclosed in PTL1 is that a sample mounting spot includes a central portion having an electrically conductive surface and a margin (peripheral) portion made of a hydrophobic mask such that the sample dripped onto the sample mounting spot crystallizes and deposits in a ring shape on the hydrophobic margin portion due to halo effect. The crystal ring formed at the margin portion is efficiently irradiated with laser light and thereby ionized.
Besides, suggestion disclosed in PTL2 is that a plurality of recessed parts each in a circular shape in a top view and called a well are provided on a sample mounting plate made of metal so that a dripped sample is dried and crystals are deposited in the recessed part. Then, preliminary measurement of the deposited sample unevenly existing in the recessed part is performed to find an irradiation site and to perform efficient ionization.
Besides, suggestion disclosed in PTL3 is that a conductive interference layer is provided on a substrate having an insulation property so as to exhibit a color different from that of the substrate, a hydrophobic layer is formed on a surface thereof, a groove forming a sample mounting spot is provided to expose the substrate, and the dripped sample is retained in the sample mounting spot (hereinafter, called an “anchoring effect”) and crystallized and ionized.